Diffuse flux of ultra-high energy photons from cosmic-ray interactions in the disk of the Galaxy and implications for the search for decaying super-heavy dark matter

TL;DR

This paper estimates the diffuse ultra-high energy photon flux from cosmic-ray interactions in the Galactic disk, highlighting its significance as a background for dark matter decay searches and emphasizing the need for increased detector exposure.

Contribution

It provides the first detailed estimate of the Galactic diffuse photon flux above 10^{17} eV, accounting for uncertainties and implications for dark matter detection.

Findings

Photon flux above 10^{17} eV is between 3.2×10^{-2} and 8.7×10^{-2} km^{-2} yr^{-1} sr^{-1}

All-sky average photon flux is about 1.1×10^{-2} km^{-2} yr^{-1} sr^{-1}

Flux decreases roughly as E^{-2} between 10^{17} and 10^{18} eV

Abstract

An estimate of the expected photon flux above $10^{17}~$eV from the interactions of ultra-high energy cosmic rays with the matter in the Galactic disk is presented. Uncertainties arising from the distribution of the gas in the disk, the absolute level of the cosmic ray flux, and the composition of the cosmic rays are taken into account. Within these uncertainties, the integrated photon flux above $10^{17}~$eV is, averaged out over Galactic latitude less than $5^\circ$, between $\simeq 3.2{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$ and $\simeq 8.7{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$. The all-sky average value amounts to $\simeq 1.1{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$ above $10^{17}~$eV and decreases roughly as $E^{-2}$, making this diffuse flux the dominant one from cosmic-ray interactions for energy thresholds between $10^{17}~$eV and $10^{18}$~eV. Compared to the current sensitivities of detection techniques, a gain between two and three orders of magnitude in exposure is required for a detection below $\simeq 10^{18}$~eV. The implications for searches for photon fluxes from the Galactic center that would be indicative of the decay of super-heavy dark matter particles are discussed, as the photon flux presented in this study can be considered as a floor below which other signals would be overwhelmed.